datasource: add testing error to make sure its the right place

pkg/datasource/datasource.go

@@ -22,6 +22,7 @@ import (
 
 var (
 	ErrNonMetricQueryNotSupported = errors.New("non-metrics queries are not supported")
+	test = errors.New("This is a test error")
 )
 
 type ZabbixDatasource struct {
@@ -147,6 +148,8 @@ func (ds *ZabbixDatasource) QueryData(ctx context.Context, req *backend.QueryDat
 					frames, queryErr = zabbixDS.queryNumericItems(queryCtx, &query)
 				case MODE_ITEMID:
 					frames, queryErr = zabbixDS.queryItemIdData(queryCtx, &query)
+				case MODE_PROBLEMS:
+					queryErr = backend.DownstreamError(test) //send a test error
 				default:
 					queryErr = backend.DownstreamError(ErrNonMetricQueryNotSupported)
 				}